Briefly, in my air pollution control process, hot product particles released from a lime reactor and used for desulfurization are quenched rapidly. Quenching induces tension at the surface and compression at the center of the particles and the tension causes cracks in the CaSO.sub.4 coating of the particle. Furthermore, compression may cause disintegration of the lime core. The particles with lime in the core are coated with cracked CaSO.sub.4 coating are called Linfans, and are suitable for desulfurization of stack gas. When Linfans are re-used or recycled for desulfurization, the lime in the core of the particles is easily reachable by the gas containing SO.sub.x through the cracks of CaSO.sub.4 coating. When the SO.sub.x in the gas is in contact with the lime in the core of the particle, the following reaction takes place: EQU SO.sub.3 +CaO.fwdarw.CaSO.sub.4
The reaction is very rapid and complete. Evidently, the sulfur trioxide is easily removed from the gas diffused into the particle, resulting in high SO.sub.3 concentration gradient in the gas in the particle and effecting a high diffusion rate of SO.sub.3 through cracks in CaSO.sub.4 coating. Thus, the SO.sub.x of the gas in the lime reactor is continuously removed.
It can be seen that the whole desulfurization process involves quenching the hot particles from lime reactor, inducing CaSO.sub.4 coating cracks, increasing reactivity of the unspent lime in the core of the particle, and recycling the reactivated lime bearing particles for further desulfurization. The use of Linfans (lime particles coated with cracked CaSO.sub.4 coating)for desulfurization has not been achieved or attempted before, and is new.
The production of "Linfans H" material from my desulfurization process can be described as follows: The hot particles from lime reactor used in desulfurization are quenched rapidly by water, steam, pressured steam, or moist air. Quenching with water having high heat capacity and high heat of vaporation will result in the creation of cracks of CaSO.sub.4 coating. As a result, the lime in the core of the particle is reachable by water, steam or moistened air through the cracks. Thus, during the quenching process, hydration of lime also takes place in the core of the particle and the reaction can be expressed as follows: EQU CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2, .DELTA.H=-19.4 KCal/Mole
The intense chemical heat generated from the hydration process is temporarily prevented from dissipation to the surrounding environment by the heat insulated CaSO.sub.4 coating, thus resulting in a sudden rise of temperature of the interior of the particle, which in turn, causes the particle to expand. The core of the hydrated lime becomes a very porous material, having greater surface area to unit weight ratio and greater reactivity. The particles having a porous hydrated lime core and cracked CaSO.sub.4 coating can be advantageously used for desulfurization of stack gas; for the hydrated core of of the particle is reachable by the gas containing SO.sub.x through the cracks of CaSO.sub.4 coating. In a hot-dry desulfurization process, when the particles are added to a hot reactor environment, dehydration of hydrated lime in the core of the particle takes place according to the following formula: EQU Ca(OH).sub.2 .fwdarw.CaO+H.sub.2 O, .DELTA.H=19.4 KCal/Mole
It can be seen that after the water is driven from the hydrated lime lattice in the dehydration process, the resulting quicklime in the core of the particle is even more porous than that of the original hydrated lime, and is also more reactive with SO.sub.x in the desulfurization process. The rate of dehydration is expected to be rapid, for the rate of dehydration is dependent on the sizes of hydrated lime grains in the core of the particle and on the temperature in the lime reactor environment. The smaller the size of hydrated lime grain and the higher the temperature of the lime reactor environment, the faster the dehydration rate will be.
From the described process for producing Linfans H material and for desulfurization it can be seen that it involves quenching, hydration, and dehydration in sequence, and one of the new materials has a coined name "Linfans Q." Linfans Q has very porous quicklime core coated with cracked CaSO.sub.4 coating. This material had never been used previously for desulfurization of stack gas before, and is very effective in SO.sub.x including SO.sub.2, SO.sub.3 removal. The mechanism of desulfurization by "Linfans H" and its modified form "Linfans Q" are the same as that by Linfans explained previously.
"Linveins" material, i.e. calcium oxide coated with fractured CaCO.sub.3 coating can also be used for desulfurization. When "Linveins" material is added to the gas containing SO.sub.3, the CaCO.sub.3 reacts with SO.sub.3 to become CaSO.sub.4 according to the following equation: EQU SO.sub.3 +CaCO.sub.3 .fwdarw.CaSO.sub.4 +CO.sub.2
This reaction takes place on the surface of the CaCO.sub.3 coating. SO.sub.3 in the gas can seep through the cracks of "Linveins" to react with CaO in the core to form CaSO.sub.4, and the mechanism of SO.sub.x removal is similar to "Linfans".
Desulfurization by "Linveins" can also be achieved after the calcium oxide core is hydrated to become porous hydrated lime, and the desulfurization process involves quenching, hydration, dehydration, lime reaction with SO.sub.x. The mechanism is the same as that in the desulfurization with hydrated lime coated with thermal shock fractured CaSO.sub.4 coating explained previously.
When "Linfans" or "Linveins" particles, having either quicklime or hydrated lime core coated with CaSO.sub.4 or CaCO.sub.3 coating, are applied to a hot reactor environment for desulfurization, the sudden heat shock may induce fragmentation of the lime core and result in more porous lime. However, heating also causes the particle to expand, and if the CaSO.sub.4 or CaCO.sub.3 coating strength is weak, the particle will desintegrate into many tiny particles. This may present material transportation and solid material separation from the gas problems which may be undesirable in many cases. In order to prevent this, heating the particles gradually or by stages to the reactor temperature inside or outside the reactor before desulfurization is the solution.
Linfans, Linfans H, Linfans Q, and Linvein can be efficiently used for SO.sub.2 removal in other dry scrubbing processes. When they are added to the flue gas containing SO.sub.2, SO.sub.2 diffuses through the cracks of either CaSO.sub.4 or CaCO.sub.3 shell to react with lime core according to the following formula: EQU SO.sub.2 +CaO.fwdarw.CaSo.sub.4 +.beta.CaSO.sub.33 +.gamma.CaS
The extent of CaSO.sub.4, CaSO.sub.3, and CaS in the reaction product depends on reaction temperature. However, in a high temperature environment, most of the resulting product is CaSO.sub.4. Since the chemical heat generated from this reaction is high, and can not be easily dissipated within the shell, therefore, the resulting product from the chemical reaction is expected to be CaSO.sub.4. The reaction mechanism of SO.sub.2 removal in the particle having a cracked shell is the same as that of SO.sub.3 removal which has been explained previously.